Artificial latex

Reclaimed mbber, which is widely used in dry mbber, has Htde use in latex compounding. A dispersion or artificial latex of the red aim must be made by a rather expensive process of milling in dispersing agents, eg, soaps and casein, and water. Some reclaim dispersions are used in latex compounds for such things as spread mbber goods and adhesives and fiber binders to reduce cost. However, for most latex compounds, it is not desirable because of the poor physical properties it imparts and the resultant darkening of the compound. 

This method was first reported by Vanderhoff [82] for the preparation of artificial latexes. The polymer and drug are dissolved or dispersed in a volatile water-immiscible organic solvent, such as dichloromethane, chloroform, or ethyl acetate. This is emulsified in an aqueous continuous phase containing a surfactant, such as poly(vinylalcohol), to form nanodroplets. The organic solvent diffuses out of the nanodroplets into the aqueous phase and evaporates at the air/water interface, as illustrated in Figure 6. The solvent is removed under reduced pressure. The nanodroplets solidify and can be separated, washed, and dried to form a free-flowing powder. 

Latex is a dispersion of polymer particles in a liquid medium, where the particles will remain suspended indefinitely. This property means that latices are colloidal dispersions. By nature of its origin, latex is classified into natural latex for dispersions obtained from plants, and synthetic latex for dispersions that are man made, typically by a process called emulsion polymerization. Blackley discusses a number of further classifications including artificial latex for dispersions in which the polymer is dispersed after synthesis, and modified latex where a chemical modification of existing latex is made. 

Direct Emulsification. Polymer colloids called "artificial latexes" can be prepared by dispersion of bulk polymers or polymer solutions into an aqueous medium. Direct emulsification processes are reviewed by ElAasser (23). The preparation procedures involve mechanical dispersion that may be followed by removal of solvent. According to ElAasser "the efficiency of emulsification," and hence the particle size characteristics of the latex, "is determined by the efficiency of formation of fine droplets and the efficiency of stabilization of the formed droplets." Important parameters in the process include the source of energy or agitation, its intensity, and duration type and concentration of emulsifiers mode of addition of emulsifier and the two phases density ratio of the two phases temperature and the rheology of the two phases. 

Latex-based polymer materials can be either nature-made, as natural rubber [Stern, 1967 White, 1995], or synthetically made. The synthetically made latexes are commonly based on recipes of monomer, water, surfactant, and free radical initiator to induce chain polymerization [Lovell and El-Aasser, 1997 Wickson, 1993], However, recipes based on step polymerization are also well known, often resulting in crossUnked films [Walker and Shaffer, 1996], The resulting latex material consists of small particles, usually spherical, of 50-500 nm in diameter, dispersed in water. Alternately, polymers are sometimes emulsified after polymerization (direct emulsification, the product sometimes called artificial latexes) via agitation of a melt in the presence of water and surfactant (emulsifier), and sometimes organic solvent or plasticizer [Piirma, 1989]. 

This exclusion of polymers from the interior of the vesicles results in an osmotic compression of the water layers and a decrease in the water layer thickness and lamellar phase volume. This effect allows the control of bulk properties such as viscosity and also provides a probe of water layer dimensions in lamellar dispersions. The lamellar surfactant system used in this study is the sodium dodecyl sulfate (SDS)/dodecanol (Ci20H)/water system that has been used to prepare submicron diameter emulsions (miniemulsions) from monomers for emulsion polymerization (5) and for the preparation of artificial latexes by direct emulsification of polymer solutions such as ethyl cellulose (4). This surfactant system forms lamellar dispersions (vesicles) in water at very low surfactant concentrations (< 13 mM). 

Artificial latex is a water dispersion of reclaimed rubber. Water dispersions of crude or reclaimed rubbers are produced by swelling and dissolving the rubber in an organic solvent, treating with an organic acid or with ammonia, and emulsifying. They resemble natural rubber latex but are softer and tackier and are used for adhesives. 

Hybrid (or composite) latexes (169) are essentially a combination of the artificial latex and emulsion polymerisation methods (68, 167). A water-insoluble species (such as polymer) may be dissolved in monomer and dispersed in water in the same marmer as the artificial latexes. However, rather than removing the monomeric solvent, it is polymerised in the droplets by the addition of initiator. The monomer-swollen polymer particles capture radicals and polymerise to form a polymeric blend or structured domains. In this maimer, polystyrene particles with styrene-butadiene mbber (SBR) inclusions have been prepared for impact modification applications. 

Artificial latexes may be formed without any polymerisation by first dissolving the bulk polymer in... 

Solvated rubbery 1,4 cis-polyisoprene polymer dissolved in hydrocarbon cement solutions are emulsified into oil-in-water emulsions with the aid of anionic surfactants and are converted to artificial latices by partitioning the emulsion through a membrane of selective permeability to the hydrocarbon solvent relative to both water and the polymer. This allows for the selective removal of hydrocarbon solvent from the emulsion, thus producing an artificial latex. The difficult problems associated with solution foaming often associated with removing hydrocarbon solvent from aqueous anionic emulsions by conventional means are thus avoided. Membrane vapour... 

The diffusion and reaction of amino-telechelic polybutadiene in poly(styrene-n-butyl acrylate/ TMI)(PSBT) was studied. Amonodisperse seed latex was prepared by semicontinuous emulsion polymerisation. Two core-shell latices were also prepared semicontinuously, using the seed latex as the core and poly(styrene-n-butyl acrylate) as the shell. These monodisperse latices were mixed with equivalent amounts of the amino-telechelic polybutadiene artificial latex before casting into films. Consumption of the TMI (dimethyl meta-isopropenyl benzyl isocyanate) in these films was monitored by FTIR as a function of time and the amino/TMI ratio. 6 refs. 

Fig. 2. Family tree of polymer dispersions. The time axis in this graph denotes the time when the different kinds of pol3mier dispersions came to awareness of mankind starting with natural latex and ending with recent developments in polymeric colloidal complexes. Some important historic milestones are the following the biosynthesis of natural rubber in plants takes place on earth since millions of years, the first heterophase polymerization was mentioned in a patent filed in 1912 (21), the first process description to make an artificial latex was published in 1923 (40), and the first block copolymer dispersion made by heterophase polymerization was described in 1952 (41).

Latex is a stable dispersion of a polymeric material (Table 8.13) in an essentially aqueous medium. An emulsion is a stable dispersion of two or more immiscible liquids held in suspension by small percentages of substances called emulsifiers. In the adhesives industry, the terms latex and emulsion are sometimes used interchangeably. There are three types of latex natural, synthetic, and artificial. Namral latex refers to the material obtained primarily from the rubber tree. Synthetic latexes are aqueous dispersions of polymers obtained by emulsion polymerization. These include polymers of chloroprene, butadiene-styrene, butadiene-acrylonitrile, vinyl acetate, acrylate, methacrylate, vinyl chloride, styrene, and vinylidene chloride. Artificial latexes are made by dispersing solid polymers. These include dispersions of reclaimed rubber, butyl rubber, rosin, rosin derivatives, asphalt, coal tar, and a large number of synthetic resins derived from coal tar and petroleum. ... 

Fig. 3. Constmction of (a) pile fiber held in place by latex (tufted) and (b) pile fiber tied into backing (knitted) grass-like artificial surfaces.

The rubber may be natural, in which case the latex is produced by the rubber tree. Latex of the main synthetic rubbers is produced by the technique of emulsion polymerisation. The term latex has been broadened in recent years and a general definition is now a stable dispersion of a polymeric substance in an aqueous medium . Latices may be classified as natural (from trees and plants), synthetic (by emulsion polymerisation) and artificial (by dispersion of the solid polymer in an aqueous medium). They may also be classified according to the chemical nature of the polymer, e.g., SBR, nitrile, polychloroprene, etc. 

Gutta percha objects were created from raw, untreated latex that was melted and molded, or from vulcanized gutta percha, which has most of the same properties as hard rubber. Molded gutta-percha was made into a staggering diversity of objects, including golf balls, jewelry, thread, tool and weapon handles, condoms, water pipes, buttons, and in dentistry as a base for dentures, as fillings, and as artificial teeth. The thread was woven into waterproof blankets, tents, and other fabrics, mosdy for use by the military. 

Sample Preparation. The polystyrene spheres to be used should be monodis-perse with a particle radius i of about 50 nm, although any size in the range i = 30 to 100 nm is suitable. Such nanospheres are available commercially as aqueous latex suspensions with 1% to 10% PS by weight. A small amount of this latex suspension should be diluted 100- to 1000-fold. Using a microsyringe, take 0.1 mL from the PS stock, deliver this into a rinsed dilution bottle, and then add 10 mL of a hltered lO-mM solution of NaCl or other 1 1 electrolyte. The purpose of this electrolyte is to partially suppress coulombic interactions (electrostatic double-layer repulsion) that can influence the diffusion constant and lead to R values that are artificially high by —10%. The electrolyte solution should be prepared from distilled water and stored at room temperature. Before use, it must be hltered through a suitable membrane (0.1-jum pore size) to remove dust particles. Avoidance of dust is cracial, and capped dilution bottles should be used. 

The simplest photonic crystal is nature s opal. The artificial opal is composed of monodispersed spheres of a dielectric, usually silica. Considerable work has been done using latex or polystyrene spheres, but we largely will restrict ourselves here to ceramics. In producing high-quality photonic crystals, care must be taken in each of the three main steps particle synthesis, sedimentation, and sintering. 

C-732 TM for ageing effects of artificial weathering on latex sealants. C-733 TM for volume shrinkage of latex sealants. 

Propylene glycol is commonly used to make antifreeze and deicing solutions for cars, airplanes, and boats to make polyester compounds and as solvents in the paint and plastics industries. It is used as a substitute for ethylene glycol mono-alkyl ethers in all-purpose cleaners, coatings, inks, nail polish, lacquers, latex paints, and adhesives. It is also used to create artificial smoke or fog used in fire-fighting training and in theatrical productions. 

---